Recent advances in technology have resulted in production of improved electronic thermometers for measuring a patient's temperature. Typically such systems include the electronic thermometer unit and a plug-in temperature sensing probe unit which may be used with disposable covers. These systems are initially relatively expensive but over their useful life they cost considerably less to use and are less consuming of the time of scarce and expensive medical personnel. However, they are more complex than conventional thermometers and so require some means for simple and easy periodic verification of their accuracy. This is especially so where one system may be used to measure temperatures of dozens of patients by virtue of the use of disposable prove covers. Further, medical personnel have neither the ability nor the time to verify the accuracy or proper operation of a suspect thermometer system. Therefore the suspect system must be removed from service to await a field service technician or return to the manufacturer.